The treatment of paper with isocyanates is known. For example, a process for treating paper with isocyanates is disclosed in EP 140537. In U.S. Pat. No. 4,505,778, a process for applying an aqueous emulsion of an aromatic isocyanate to a wet paper web is disclosed. However, the application of isocyanates to paper by conventional methods has inherent disadvantages.
Isocyanates emulsified with water have conventionally been applied to paper using a size press and unemulsified isocyanates have been applied using a rotogravure coating machine. When conventional means of spraying isocyanates are used, they are characterized by low transfer efficiencies. As a result, significant amounts of isocyanate are released into the atmosphere, thereby creating potential health problems. In addition, when the isocyanate that is applied to paper is not sprayed, solvent diluted, or emulsified, it is very difficult to apply the isocyanate at levels below 5 to 10 percent by weight based upon the weight of the paper. Lower isocyanate levels can be achieved by using emulsified isocyanates. However, pot life problems with the isocyanate can arise when the isocyanates are emulsified since the water begins to react with the isocyanate groups. Furthermore, depending upon the stage of the process at which the emulsion is added, it can result in additional drying requirements. There is therefore a need for a process for efficiently applying isocyanates to paper substrates at levels below 5 percent by weight based upon the weight of the paper substrates and a process that does not require the use of emulsified isocyanates, although emulsifiable isocyanates may be used.
Although the application of isocyanates both strengthens and sizes paper, concerns have arisen regarding possible adverse health effects which may result from isocyanate which may extract from the paper. Such concerns are particularly relevant to paper which may come into contact with food. Thus, the Food and Drug Administration (FDA) has set guidelines for amounts of any additive which may extract from paper which may be repulped into food packaging or wrap. Therefore, there also exists a need for a process which reduces the amount of isocyanate which extracts from the paper product(s) to which it is applied.
Surprisingly it has been found that these needs can be met by using an electrostatic spraying device to apply isocyanates to paper. By using this electrostatic spraying device, transfer efficiencies above 90 percent can be achieved and the isocyanate can easily be applied to paper substrates at levels well below 5 percent by weight. The amount of isocyanate needed according to the invention can be as low as 0.5 g/m.sup.2. However, in commercial production settings, the amount of isocyanate applied is generally about 1.0 to 1.5 g/m.sup.2.
Electrostatic spraying devices are known (see, for example, U.S. Pat. Nos. 4,854,506 and 4,846,407 and EPA-193348). The electrostatic spraying of various polymeric materials onto paper has also been disclosed in U.S. Pat. Nos. 3,930,614, 4,609,686 and 4,837,057. However, none of these patents disclose the electrostatic spraying of isocyanates onto paper.
It is therefore an object of this invention to provide a means for efficiently applying an isocyanate to a paper substrate.
It is a further object of this invention to provide a means for improving the crush strength, water resistance and wet strength of paper products.
It is an even further object of this invention to provide a means of applying a uniform coating of isocyanate onto a paper substrate.
It is yet another object to apply an isocyanate to paper in an environmentally safe manner.
It is a further object of the present invention to apply an isocyanate to paper in such a manner so as to reduce the amount of isocyanate which may extract from the paper over time.
These and other objects are obtained by the process of this invention.